Thermoplastic polymeric films



" April 1, 1958 R; HAMICHEL 2,829,069

THERMOPLASTIC POLYMERIC FILMS Filed Nov. 22, 1954 COPOLYMER 0F 809'llo VINYLIDENE CHLORIDE AND 3-207. POLYHERIZABLE HONC-OLEFINIC NONCIIER.

CCPOLYHER OF UP TO 50% VINYLI- DEIIE CHLORIDE AND 50|00% ALKYL ACRYLATE.

THERMOPLASTIC POLYMERIC MATERIAL INVENTOR RUDOLPH H. MICHEL ATTORNEY lack the requisites for some United States Patent 2,829,069 THERMOPLASTIC POLYMERIC FILMS Rudolph H. Michel, Kenmore, N. Y., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application November 22, 1954, Serial No. 470,553

' r Claims. (Cl. 117-76) This invention relates to the manufacture of thermoplastic polymeric films such as films of polyethylene terephthalate, polyethylene, polystyrene, rubber hydrochloride and polyvinyl chloride. More, particularly, it relates to the preparation of polyethylene terephthalate and polyethylene films suitable for conversion to bags, containers and similar packages.

The desired properties for transparent packaging films are well konwn. They include strength, heat-sealability, durability of the heat-seal as well as durabilityof the complete film, moisturcproofness, low permeability to oxygen and carbon dioxide, high gloss, clarity and freedom from electrostatic charges. I

In the case of thermoplastic polymeric materials such as polyethylene terephthalate, strength is obtained by.

orienting the film in a stretching and/ or rolling operation followed by heat-setting the film. Orienting, however, increases the degree of crystallinity and effects other microstructural changes which tend to impair some of the remaining properties of the film. For example, after orienting polyethylene terephthalate film, it is no longer possible to heat-seal the film with most conventional heat-sealing apparatus and obtain durable bonds. Attempting to heatseal results in severe shrinkage with attendant puckering and weakness at the seal.

On the other hand, thermoplastic 'materials such as polyethylene do not require orienting to obtain strength. As unoriented films they possess properties which contribute to their successful utilization in a wide variety of packaging applications. However, polyethylene films packaging applications particularly vacuum packaging. Limpness (lack of stiffness) and permeability to such gases as oxygen, carbon dioxide,

and organic vapors preclude the use of polyethylene films for vacuum packages.

Recognizing the above major obstacles to wider commercial use of thermoplastic polymeric films such as polyethylene terephthalate and polyethylene, the industry has proposed various remedies. form of coatings or other treatments which tend to improve one property at the expense of another property. However, a thermoplastic polymeric film possessing the optimum requirements for use in a wide variety of commercial packaging applications has remained an elusive goal.

These remedies take the The object of the present invention is to provide a novel thermoplastic polymeric film fulfilling the requirements of 'a packaging material, i. e. strength, durability, heat-.

scalability,

etc. Another object is to specify a process for preparing moistureproofness, low permeability to gases,

the novel thermoplastic polymeric films, particularly films of polyethylene terephthalate and polyethylene. Other objects will appear hereinafter.

The objects are accomplished by providing a base film of thermoplastic polymeric material having at least one surface coated with two special coatings:

(1) A subcoating comprising a copolymer obtained by vinylidene chloride and 50100% 2,829,069 Patented Apr. 1,

polymerizing from -97% vinylidene chloride and 3- 20% of at least one-other polymerizable mono-olefinic monomer copolymerizable therewith, preferably an alkyl;

acrylate. v v

The accompanying drawing illustrates m a crosssectional view thenovel packaging film of this invention.

mono-olefinic monomer are identified by legends.

Preferably, the subcoating comprises a copolymer obtained from 2030% of. vinylidene chloride, 7080,% of an alkyl acrylate, and 0.5 %-3 by weight, based upon the total weight of the previous two components, of an unsaturated aliphatic acid from the group consisting of acrylic, methac'rylic, and itaconic acids, and the top coating comprises a copolymer obtained from -97% vinylidene chloride, 3-10% of an alkyl acrylate, and 0.5%3% by weight, based upon the total weight of the previous two components, of an unsaturated aliphatic acid from the group consisting of acrylic, methacrylic and itaconic acids.

In a specific embodiment of this invention, a base film of polyethylene terephthalate, 0.5 mil thick stretched three times its original dimensions in two directions and heat set at 200 C. while under tension, was coated with the two special coatings. The coatings were applied as dispersions prepared by AB, ammonium persulfate, itaconic acid and water to a vessel fitted with a stirrer and a reflux condenser. After dissolving these components by stirring, vinylidene chloride and the alkyl acrylate were introduced and the mixture refluxed at 35 C. A solution of metasodium bisulfite was added and the mixture stirred until refluxing ceased, thereby indicating completon of polymerization. A quantity of Duponol, e. g., 7.3 pounds of Duponol WAQ for the charge given below or 2.25 pounds of Sulframin AB in water may be added subsequent to polymerization to stabilize the copolymer against coagulation.

itaconic acid was prepared from the'following charge:

Water 240 lbs. Duponol WAQ 7.3 lbs. Ammonium persulfate 152 grams. 'Vinylidene chloride 34 lbs.

2-vethylhexyl acrylate 78 lbs. Itaconic acid 1.12 lbs. Meta-sodium bisulfite 76 grams in 1.5 lbs.

' of water. Added subsequent to polymerization: Su1framinAB 2.25 lbs. in 10 lbs. of water.

The top coat containing a copolymer of 92% vinylidene chloride, 8%

the total weight of the previous two components, of itaconic acld was prepared from the following charge:

Water lbs.

Duponol WAQ 10 lbs.

Ammonium persulfate 102 grams.

Vinylidcne chloride '138 lbs.

Methyl acrylate 12 lbs.

itaconic acid 1.5 lbs.

Meta-sodium bisulfite 51 grams in 1.5 lbs. of

water.

1 Sodium salt of a fatt alcohol sulfate in a e s l (33 0 active ingredient). q on so men odium salt of an alkyl benzene sulfonate,

adding Duponol WAQ, Sulframin methyl acrylate and 1%, based on' 3 Added subsequent to polymerization: Sulframin" AB "u 3 lbs. in lbs. of water.

The polyethylene terephthalate base film was subcoated by passing it through a bath .of the first dispersion. Contact time was-sufficient to allow the base film to retain a slight excess of the subcoating. The excess was removed by doctoring as the coating was made smooth and broughtto the desired thickness. The single-coated film was then dried in an atmosphere maintained at a ternpernturebetween 120 C. and 130 C. (it is important that the subcoating is fully coalesced and substandally dry before the top coating is applied.) Then the aqueous dispersion of the top coating was applied, smoothed and dried in a similar manner. The total coating weight of the coated film was 16 grams/square meter; 10 grams/square meter of the subcoating, 6 grams/square meter of the top coating.

The resulting film retained its strength, high gloss and clarity. In thefollowing table, the properties of the resulting film "are compared to those of uncoated and single-coated films.

1 Conditioned at 75 F. and 35% relative humidity.

The details of the methods for testing the films follow: Coatingweight is determined by immersing aportion of the filmin. cyclohexanone at room temperature for 15.- minutes or longer if necessary to dissolve the coating entirely; rinsing in ethyl acetate; and drying overnight at room temperature and 35% relative humidity. The

sample is weighed before and after this treatment to' I determine the coating weight.

Moisturepermeability. is determined by placing the test filmover the top of an aluminum cup containing millilitersofwater, the test area being 33.3 square centimeters. w The assembly is weighed accurately and then placed in a dry (less than 3% relative humidity) airswept oven at 395 C. for-24 hours. removed from the oven, cooled to room temperature and reweighed. The weight loss is converted to grams of water lost/100 square meters/hour.

Heat-seal strength is measuredby cutting a piece of the coated film 4" x 10" with the grain running in the long direction into two pieces 4" x 5". The two pieces are superimposed so that opposite surfaces are in corn tact. The two pieces of superimposed :film are then sealed together at each end at right angles to the grain.

For polyethylene terephthalate, a /4" wide sealing bar heated to a temperature of 160 C. at p. s. i. pressure contacts the ends for two. seconds. (For polyethylene film, the sealing. bar is Ma" wide, at a temperature of 210-230" C. at 10 p. s. i. and a 0.15 second contact time is used.) The sealed sheets are then cut in half at right angles to the grain. From the center of the four resulting pieces, 1V2" wide strips parallel, to the. grain are cut. The four sets of strips are then conditioned for one day at 75 F. and relative humidity. They are tested by opening each set of strips at the free ends, placing them in a Suter testing machine and pulling themapart. The highest force in grams required to pull the strips apart is taken as a measure of the heat-seal bond strength.

Heat-seal durability is measured by an accelerated The assembly is t as a functionof time.

durability test, the rice bag drop test. The rice' bags are made by folding a 4" x 15" sheet of the coated film and sealing the film along the two long edges with the bar type heat sealer described previously. After 100 grams of rice are put into the bag, the open end is sealed with the same type of heat sealer., This leaves a small but uniform amount of' free space above the rice. The bags represent envelopes of outside dimensions 4-" x 6 /2 and inside dimensions 3 x 5". The bags are conditioned overnight at F. or 5 F. and 35% relative humidity. The test consists of releasing the bag from a fixed clamp which is 2 feet above the floor of a porcelain tray. The bags are held by the end seal and allowed to drop onto the non-sealed bottom edge. The number of falls survived before loss of rice occurs from any type of failure, is taken as a measure of durability. The various letter designations represent the types of failure: 1

A-seal failure with no film tearing B-min0r film tearing within the heat seal during the course of its rupture C-more severe film tearing within the heat seal Dtearing at theedge'of the heat seal Film durability is evaluated for coated polyethylene film instead of heat-seal durability. The application of polymeric coatings to polyethylene film tends to degrade film durability. On the other hand, the durability of polyethylene terephthalate film is not materially. aifected by polymeric coatings. The accelerated film durability rating is obtained by folding over a 5" x 12" piece of film and sealing it along two sides with a /1" strip of a pressure sensitive cellophane tape. This provides a 5" x 6" envelope which is then filled with 400 grams of rice. The envelope is then sealed at the top with the pressure-sensitive tape, leaving a small butuniform amount of free space above the rice. Normally, the longest dimension of the piece of 5" x 12" film runs in the direction in which the film has been extruded In testing the bag, this length-wise direction of the film is perpendicular to the plane upon which the bag is dropped. The test consists of dropping the bag onto a stonetopped laboratory desk from a fixed height of 18 inches above the desk, i. e., 18 inches from the bottom of the bag to the desk top. The bags are allowed to drop onto the non-sealed (bottom) edge of the bag. The number of falls or drops survived before .1055 of rice from film failure is recorded; and tests in which the loss of rice resulted from failure of the pressure-sensitive tape are invalidated. Generally, five bags are tested for 'each film condidate, and the average taken.

Gas permeability is measured by sealing the test film between the high and low pressure side of a split permeability cell (the test area being 39 square centimeters). The test gas is passed through the high pressure side while the low pressure side, towhich a manometer is attached, is evacuated to a pressure of approximately 3 millimeters of mercury. At this point the low pressure side is sealed off but the flow of the test gas at one atmosphere is continued through the high pressure side. Diffusion of gas through the test film is then graphically recorded by plotting, the rate of fall. of the manometer Gas laws are applied to the data so obtained. and the gas permeability value is expressed in grams/ square meters/hour.

Tables H and III, which follow, summarize :the examples of this invention. Table II presents examples employing polyethylene ,terephthalate as the base film; Table III, polyethylene. Examples I4 and 10-11 are outside the scope of'the invention and are presented for comparison.

The films were prepared essentially in the manner passed over a tensioning roll into a tank containing the coating composition. The coated, film" was then passed between two doctor rolls to smooth the. coating and toregulate its thickness. The

and under guide roll;

stretched and heat-set, stretched only or neither stretched nor heat-set.

A particularly useful application of the invention is found inusing polyethylene terephthalate which has been coated film was dried in a tower maintained-at 120 C.- stretched and/or rolled but not heat-set. This twice- 130 C. in the case of coated polyethylene terephthalate coated polyethylene terephthalate film stretched three and 80 C.-85 C. for, polyethylene. The second or top times its original dimensions in two directions, is outcoating was then applied similarly. v standing as a heat-shrinkable tight wrapping for such Thecoating compositions were prepared substantially articles as frozen poultry, ham, smoked meats, etc. The as described for the specific embodiment. The amounts coated heat-shrinkable film is normally formed into a of polymer components werevaried to provide the perbag open at one end; the article of food is placed into centages givenjin the tables. the bag; and the bag is sealed. Air may be evacuated Before. applying the coating compositions to polyfrom the bag before sealing if desired. The bag contain.- ethylene' film, 'the'base film was pretreated to improve ing the article of food is then immersed into a hot liquid, adhesion of the coating compositions to the film. The e. g., water at 70-100 C., to shrink the bag around the treatment, asfdis'clo sed in 'copending application Serial article. No. 323,271, filed November 29, 1952, to-L. E. Wolinski, The coatings may also'be applied to unstretched films comprises exposing the f reshly formed film to ozone at which may then be stretched in one or two directions atemperature above 150 C. immediately after extru- (as described .in U. S. Patent 2,627,088 to Alles and sion' of the film. It} is believed .that this treatment Saner) This procedure also produces a coated, heatroughens the surface of the film. When applying the shrinkable wrapping film which may be used directly in coating compositions to polyethylene film, care must be sheet form or fabricated into bags. 7 taken to avoid stretching or elongatingthe film. The important concept leading to the Wrapping ma- In the examples, the components used for preparing terials described in the examples and the materials dethe copolymers are given in weight percentages. The. scribed in the preceding paragraphs resides in the use of percentages of itaconic acid are based on thetotal weight a base film of thermoplastic polymeric material coatedof the major components. Thus, a copolymer from with two critical coatings, namely; a subcoating comprisvinylidene chloride, 70% alkyl a'crylate and 2% ing a vcopol ymer obtained from up to 50% by weight of itaconic acid may be prepared from 60pounds of vvinylvinylidene chloride and 50-100% by weight of an alkyl idene chloride, 140 pounds of the alkyl acrylate and '30 acrylate, wherein the alkyl group contains at least2carbor1 4 pounds of itaconic acid. I I f I atoms, and a top coating comprising a copolymer obtained For convenience, abbreviations have been used in the from 80-97% vinylidene chloride and 3-20% of at least tables; 'VCl represents vinylidene chloride; 2-EHA, 2-' 1 one other polymerizable mono-olefinic monomer copolyethylhexyl acrylate; IA, itaconic acid; EA, ethyl acrylate; merizable with vinylidene chloride. MA, methyl acrylate;-qn-QA, normal-octyl acrylate; g., The alkyl acrylate in the subcoating contributes to the gramsymi, meters. I elasticity of the'subcoating; The amount of thiscomv l ,TABLE II Properties of 'polyethylene terephthalate films Subcoating Top coating Total HqO-O -CO; Heat-seal durabil- I coating permeability Heat-seal ity (falls survived) Example H 1 weight (gJiOO rnfl/ strength Composition Weight Composition Weight (gJmfi) hr.) (g./1.5in)

. g./m. 1 (g./m.*) 75 F. 5 F. v

None-. Nonett 3504.0t.7 0 0 do 90/l0/l-VOlz/EA/IA.-- 9.6 0.0 50 450 0B Mao. 90/10/1-VO12/EA/IA--. 10.4 10.4 49 415 1B 1 9O/10/1-VC12/MA/IA 14.0 14.0 690 0D 30/70/i-VCh/2-EHA/IA. 10.0 92/8/1-VC12/MA/IA 0.0 16.0 201"-1" 500 22A 20/80 .V01i/2-EHA;- 9.5 92/8/l-VClz/MA/IA 5.0 14.5 30 430 18A 100% 2EHA 1 8.0 92/8/1-VC11/MA/IA 8.0 16.0 28 450 10A 30 70 Von/molt 10.0 90/10/1-V0lz/MA/IA 6.0 16.0 30 550 15A 30 70 vols/EA- 10.0 92/8/i-VCh/MA/IA 6.0 16.0 28 400 5A 1 Conditioned at 75 F. A

b Conditioned at 5 F: fLess than 1. Y

' l I TABLE III "Properties of polyethylene film Bubcoating 'Top coating Total H:OOz-C0z Film coating permeability Heat-seal durabil- Example weight (g./100 mfi/ strength ity (falls Composition H Weight Composition Weight (gJmJ) hr.) (g./1.5 in.) survived) (s-l (el 70-12-15 1, 500 8.0 8.0 40-1-1 450 2 12 5.4 14.9 30-1"-1" 900 30* 6.8 15.8 3511 s50 35' 14 00% A 0.0 14.9 'ss-w- 020 '30 15-- 30/70-VO1i/n-0A 0.2 10.0 37.-1-1"" 950 30*. 16-0--.- 30/70-Vol2/EA 5.0 14.3 .3s-1"-1" s 20* Greater than 50, 30, etc. Less thanl.

.While the examples referto coating polyethylene tere:

The moisture impermeability of the film of this invent ponent necessary to provide a packaging material of satisfactorystrengthand durability is dependent upon the par- I ticular a1kyl acrylate used.

1 tion is contributed by the top coating composition. A hard polymerprepared from at least 80% vinylidene chloride and 320% of at least one other polymerizable mono-olefinic monomer is necessary. As polymerizable monomers for use with the vinylidene chloride in the top coating, I have illustrated the use of alkyl acrylates. However, the invention is not limited to these. Other polymerizable monomers may also be used. The list in cludes: methyl, ethyl, isobutyl, butyl, octyl and Z-ethylhexyl. acrylates and methacrylates; phenyl methacrylate, cyclohexyl methacrylate, p-cyclohexylphenyi methacrylate, methoxyethyl methacrylate, chloroethyl methacrylate, 2- nitro-Z-methylpropyl methacrylate, and the corresponding esters of acrylic acid; methyl alphachloroacrylate, octyl alpha-chloroacrylate, methyl isopropenyl ketone, acrylonitn'le, methacrylonitrile, methyl vinyl ketone, vinyl chloride, vinyl acetate, vinyl propionate, vinyl chloroacetate, vinyl bromide, styrene, vinyl naphthalene, ethyl vinyl ether, N-vinyl phthalimide, N-vinyl succinimide, N-vinyl carbazole, isopropenyl acetate, acrylarnide, methacrylamide or monoalkyl substitution products thereof, phenyl vinyl ketone, diethyl fumarate, diethyl maleate, methylene diethyl malonate, dichlorovinylidene fluoride, dimethyl itaconate, diethyl itaconate, dibutyl itaconate, vinyl pyridine, maleic anhydride, allyl glycidyl ether and other unsaturated aliphatic ethers described in U. S. Patent 2,160,943. These compounds may be described as vinyl or vinylidene compounds having a single CH =C group. The most useful ones fall within the general formula where R may be hydrogen, a halogen or a saturated aliphatic radical and X is selected from one of the following groups: -Cl, Br, F, -CN, C H COOH,

OR -OC H CONH CONH-R', arid -CONR in which R is alkyl.

The coatings may be applied from aqueous or organic vehicles, i. e., in the form of aqueous dispersions or from solutions of the polymers in organic solvents. For optimum properties, unsaturated aliphatic acids such as ita conic acid, acrylic acid and methacrylic acid may be incorporated in the coating compositions. amples illustrate the use of itaconic acid in the coating compositions, substantially equivalent quantities of either acrylic acid or methacrylic acid used in place of itaconic acid produce similar results. When the compositions are applied from organic solutions, the acid may be omitted with little deleterious effect. When coating from aqueous dispersions, the acid may also be omitted if steps are taken to facilitate wetting of the film by using commercial wetting agents or other methods. It should be pointed out that films coated with compositions that do not include the unsaturated aliphatic acids are somewhat inferior in the degree of transparency. For this reason, it is pre- While the exferred to include one of the above unsaturated aliphatic acids in both the suband top coating compositions.

The coatings may be applied in accordance with any known coating techniques. They may be applied by passing the film through baths in a continuous manner or in-a batch manner. Coatings may also be sprayed on the film, or applied manually by brushing or the like. The thickness of the coatings may be adjusted in accordance with-method well known in the coating art. Selection of the proper thicknesses of the top coating and the subcoating depends to some extent on the properties that one desires to emphasize. Thus, the thickness of the subcoating influences the strength and durability of the heatseal and the durability of the film in general. The thickness of the top coating influences the permeability of the film to moisture and gases.

The double-coated films of this invention are useful as packaging materials for foods, cigarettes and the like. Their durability and their impermeability to gases and water vapor combine to provide wrapping materials that can withstand the abuse of handling and shipping and wherein the freshness of the products may be retained over long periods. A particularly useful application of the improved double-coated polyethylene terephthalate film is in refrigerating units. Polyethylene terephthalate film, being impervious to halogen-substituted hydrocarhon gases such as dichlorodifluoromethane, can be used in the units wherever insulation from these gases is neces- SRI'V.

While the invention contemplates the use of the copolymers as the essential constituents of the coatings, other ingredients may be added. The addition of pigments, dyes, delustrants, plasticizers, etc., is therefore understood to be within the purview of this invention.

As many Widely different embodiments may be made without departing from the spirit and scope of this invention, it is understood that the invention is not limited except as defined in the appended claims.

What is claimed is:

l. A packaging film comprising base film of thermoplastic polymeric material having at least one surface coated with a subcoating comprising a copolymer obtained from up to vinylidene chloride and 50100% alkyl acrylate wherein the alkyl group contains at least two carbon atoms, and a top coating comprising a copolymer obtained from 80-97% vinylidene chloride and 320% of at. least one other polymerizable mono-olefinic monomer copolymerizable therewith;

2. A packaging film comprising base film of thermoplastic polymeric material having at least one surface coated with a subcoating comprising a copolymer obtained from 20-30% vinylidene chloride and -80% of an alkyl acrylate wherein the alkyl group contains at least two carbon atoms, and a top coating comprising a eopolymer obtained from 9097% vinylidene chloride and 310% of at least one other polymerizable monoolefinic monomer copolymerizable therewith.

3. A packaging film comprising a base film of thermoplastic polymeric material having at least one surface coatedwith a subcoating comprising a copolymer obtained from up to 50% vinylidene chloride, 50-l00% alkyl acrylate wherein the alkyl group contains at least two carbon atoms and 05-23%, based on the total weight of the previous components, of an unsaturated aliphatic acid from the group consisting of acrylic acid, methacrylic acid and itaconic acid and a top coating comprising a copolymer obtained from -97% vinylidene chloride, 3-20% alkyl acrylate and 0.5-3%, based on the total weight of the previous components, of an unsaturated aliphatic acid from the group consisting of acrylic acid, methacrylic. acid and itaconic acid.

4. A packaging film comprising a base film of polyethylene terephthalate having at least one surface coated with a subcoating comprising a copolymer obtained from up to 50%. vinylidene chloride, 50-l0()% alkyl acrylate wherein the alkyl group contains at least two carbon atoms and 0.53%, based on the total weight of the previous components, of an unsaturated aliphatic acid from the group consisting of acrylic acid, methacrylic acid and itaconic acid and a top coating comprising a copolymer obtained from 80-97% vinylidene chloride, 3-20% alkyl acrylate and 0.53%, based on the total weight of the previous components, of an unsaturated aliphatic acid from the group consisting of acrylic acid, methacrylic acid and itaconic acid.

5. A packaging film comprising base film of polyethylene having at least one surface coated with a subcoating comprising a copolymer obtained from up to 50% vinylidene chloride, 50-100% alkyl acrylate wherein the alkyl group contains at least two carbon atoms and 0.5-3%, based on the total weight of the previous components, of itaconic acid and a top coating comprising a copolymer obtained from 8097% vinylidene chloride, 33-20% alkyl acrylate and 0.5-3%, based on the total weight of the previous components, of itaconic acid.

6. A film as in claim. 3 wherein the alkyl acrylate in the top coating is methyl acrylate.

7. A film as in claim 1 wherein the alkyl acrylate in the subcoating is 2-ethylhexyl acrylate.

8. A film as in claim 1 wherein the alkyl acrylate in the subcoating is ethyl acrylate.

9. A film as in claim 1 wherein the alkyl acrylate in the subcoating is n-octyl acrylate.

10. A film as in claim 3 wherein the unsaturated aliphatic acid in the coatings is itaconic acid.

11. A process for preparing a packaging film which comprises coating a base film of thermoplastic polymeric material with a copolymer obtained from up to 50% vinylidene chloride, 50-l00% alkyl acrylate wherein the alkyl group contains at least two carbon atoms and 0.5-3 based on the totalweight of the previous components, of itaconic acid; drying the coated base film; applying to the dried coated film a second coating comprising a copolymer obtained from 80-97% vinylidene chloride, 3-20% alkyl acrylate and 0.5-3%, based on the total weight of the previous components, of itaconic acid; and drying the twice-coated film.

12. A process for preparing a packaging film which comprises coating oriented polyethylene terephthalate film with a copolymer obtained from vinylidene chloride, Z-ethylhexyl acrylate and 1%, based on the weight of vinylidene chloride plus 2-ethylhexyl acrylate, of itaconic acid; drying the coated film; applying to the dried coated film a second coating comprising a copolymer obtained from 92% vinylidene chloride, 8% methyl acrylate and 1%, based on the weight of vinylidene chloride and methyl acrylate, of itaconic acid; and drying the twice-coated film.

13. A process for preparing a packaging film which comprises coating a base film of thermoplastic polymeric material with copolymers obtained from 50% vinylidene chloride and 50-100% alkyl acrylate wherein the alkyl group contains at least two carbon atoms in a suitable medium; drying the coated base film; applying to the dried coated film a second coating comprising a copolymer obtained from -97% vinylidene chloride and 3-20% of at least one other polymerizable mono-olefinic monomer copolymerizable with vinylidene chloride in a suitable medium; and drying the twice coated film.

14. A process as in claim 13 wherein the thermoplastic polymeric material is polyethylene terephthalate.

15. A process as in claim 13 wherein the thermoplastic polymeric material is polyethylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,627,088 Alles et a1. Feb. 3, 1953 FOREIGN PATENTS I 500,178 Canada Feb. 23, 1954 

1. A PACKAGING FILM COMPRISING BASE FILM OF THERMOPLASTIC POLYMERIC MATERIAL HAVING AT LEAST ONE SURFACE COATED WITH A SUBCOATING COMPRISING A COPOLYMER OBTAINED FROM UP TO 50% VINYLIDENE CHLORIDE AND 50-100% ALKYL ACRYLATE WHEREIN THE ALKYL GROUP CONTAINS AT LEAST TWO CARBON ATOMS, AND A TOP COATING COMPRISING A COPOLYMER OBTAINED FROM 80-97% VINYLIDENE CHLORIDE AND 3-20% OF AT LEAST ONE OTHER POLYMERIZABLE MONO-OLEFINIC MONOMER COPOLYMERIZABLE THEREWITH. 